The present invention relates to load transfer systems and more particularly, to cargo landing systems.
The need for safe and efficient cargo transfer between ships at sea has increased in recent years. Two areas where this need has been particularly felt are offshore oil drilling and mining, and naval operations. In these areas, the difficulty in transferring cargo even in moderate sea conditions has proven to be a severe operational limitation. In these examples where a load must typically be off-loaded onto a deck which moves with respect to the load support, it is important to control the relative motion of the load to provide a minimal velocity difference impact with the deck.
Prior art techniques for rough sea off-loading of cargo onto drill ships and oil platforms have been generally unsuccessful because the conventional crane equipment was not designed to withstand the stresses introduced by the dynamic conditions, particularly introduced by violent heave motions. Newly developed pressure compensated crane equipment has been effective to accommodate such conditions to a degree, although this approach provides only open-loop corrections for the extremely complicated dynamics presented by a rough sea environment.
With conventional techniques, operator-controlled crane equipment has been successful in matching sea-induced horizontal relative motions of the landing platform and the load. However, conventional equipment has been unable to accommodate violent heave motions in rough seas. Violent heave motions pose two problems in particular for transfer operations. The first problem, dynamic loading on the structure due to vertical accelerations, has been approached in the prior art generally through the use of pressure compensated control equipment and redesign of the load bearing structural elements. The second problem is more significant and relates to the relatively large heave velocities that have to be accommodated when transferring loads between two moving ships, or between a ship and a platform. Any large uncompensated relative motion at impact can result in potentially costly cargo damage.
In the prior art, a load synchronization approach to this problem uses relative velocity information to synchronize the load motion with the relative heave motion. By introducing a velocity bias, a constant closing rate between the load and deck may be established. Typically, the load synchronization technique uses the sensed velocity difference between cargo load and receiving deck to adjust the load line velocity in order to maintain a roughly constant, small closing velocity until impact. The power required to implement a load synchronization controllers scales directly with the cargo mass and the relative velocity between the platform. When the mass or relative motion rates are large, the power requirements are correspondingly large.
The load-synchronization technique has several advantageous features: first of all, such techniques require only a relative velocity measurement which can be obtained by conventional Doppler, optical or mechanical techniques. In addition, synchronization hardware can be mechanized with relative ease. For example, simple swash-plate angle control on a variable-displacement pump may be utilized to slave the load motion with respect to the off-loading platform at any desired rate. However, the synchronization techniques also are characterized by offsetting disadvantages, including the consumption of large amounts of power in cases where the cargo is heavy, or when relative-motion disturbances command high rates. Another disadvantage is the requirement to accommodate severe structural loading.
It is an object of the present invention to provide a system for landing a cargo on a deck with minimal velocity difference at impact.
Another object is to provide an adaptive cargo landing system having a closed loop variable gain controller.